The invention relates to an electrodeless high-pressure discharge lamp comprising:
a light-transmitting lamp vessel which is closed in a vacuum tight manner and which has an ionizable filling;
around the lamp vessel, an electric coil having turns along a plane through the lamp vessel, which coil has end portions which are electrically connected to current conductors which are to be connected to an electric supply.
Such an electrodeless high-pressure discharge lamp is known from U.S. Pat. No. 5,042,139.
The coil of the known lamp is built up from voluminous, for example solid, turns. The result of this is that a comparatively large surface area of the lamp vessel is screened off from its surroundings. Light generated in the lamp vessel as a result cannot freely emerge, which reduces the luminous efficacy of the lamp. This disadvantage also holds for a coil whose upper and lower turns have a conical upper and lower surface, respectively.
It is to be prevented that the coil assumes a comparatively high temperature, and thus a comparatively high electric resistance, owing to current passage and radiation from the lamp vessel. A higher electric resistance would cause the ohmic losses to increase, and as a result also the temperature. To remove heat from the coil, the known coil may have hollow turns through which water is circulated. The screening of generated light, however, is not counteracted by this modification, while the modification has the disadvantage of additional provisions, i.e. the water supply and drain, as well as the energy consumption thereof.
U.S. Pat. No. 4,910,439 discloses an electrodeless high-pressure discharge lamp of the kind mentioned in the opening paragraph in which a forced air current cools the electric coil. Apart from the screening of the lamp vessel, this lamp has the disadvantage that a motor and supply lines are necessary for cooling, and also that energy is required for this.
A disadvantage of a mechanical cooling is, furthermore, that maintenance is required for it and that the life of the cooling drive may be the factor which limits lamp life in the case of a lamp capable of burning a few tens of thousands of hours.
U.S. Pat. No. 4,871,946 discloses an electrodeless high-pressure discharge lamp whose coil is helicoidally wound against the lamp vessel. Here the coil not only intercepts light, but is also strongly heated by the discharge vessel, whereby its resistance increases.
Copper is particularly suitable as a material for the coil because of its high electrical conductivity. Copper has the disadvantage, however, that it readily oxidizes at increased temperature and then turns black. A voluminous coil around the lamp vessel will then not only intercept light, but also absorb it.
GB 2,217,105 discloses an electrodeless high-pressure discharge lamp in which a coil is wound helicoidally around the lamp vessel and has a light-reflecting coating. This only achieves, however, that incident light is partly reflected. Silver, which has a comparatively high reflectivity, however, quickly assumes a dark colour at elevated temperature owing to oxidation. Chromium is comparatively oxidation-resistant, but it has a comparatively low reflectivity. Coatings of these metals, accordingly, are not effective.